John Day Data Entry Terminal Option
June 27,1977 NIC 40652
RFC 731
Telnet Data Entry Terminal Option
1. Command Name and code:
DET 20
2. Command Meanings
IAC WILL DET
The sender of this command REQUESTS or AGREES to send and
receive subcommands to control the Data Entry Terminal.
IAC WONT DET
The sender of this command REFUSES to send and receive
subcommands to control the Data Entry Terminal.
IAC DO DET
The sender of this command REQUESTS or AGREES to send and
receive subcommands to control the Data Entry Terminal.
IAC DONT DET
The sender of this command REFUSES to send and receive
subcommands to control the Data Entry Terminal.
The DET option uses five classes of subcommands 1) to
establish the requirements and capabilities of the
application and the terminal, 2) to format the screen, and
to control the 3) edit, 4) erasure, and 5) transmission
functions. The subcommands that perform these functions are
described below.
T__h_e_N__e_t_w_o_r_k_V__i_r_t_u_a_l_D__a_t_a_E__n_t_r_y_T__e_r_m_i_n_a_l(NVDET)
The NVDET consists of a keyboard and a rectangular
display. The keyboard is capable of generating all of
the characters of the ASCII character set. In addition,
the keyboard may possess a number of function keys which
when pressed cause a FN subcommand to be sent. (Although
most DET's will support one or more peripheral devices
such as a paper tape reader or a printer, this option
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does not consider their support. Support of peripheral
devices should be treated by a separate option.)
The screen of the data entry terminal is a rectangle M
characters by N lines. The values of M and N are set by
negotiating the Output Line Width and Output Page Size
options, respectively. The next writing position (x,y)
on the screen (where x is the character position and y is
the position of the line on the screen) is indicated by a
special display character called the cursor. The cursor
may be moved to any position on the screen without
disturbing any characters already on the screen. Cursor
addressing in existing terminals utilizes several
topologies and addressing methods. In order to make the
burden of implementaton as easy as possible this protocol
supports two topologies (the finite plane and the helical
torus) and three addressing methods ((x,y); x and y, and
relative increments). Since the finite plane with
absolute addressing is the least ambiguous and the
easiest to translate to and from the others, it is the
default scheme used by the NVDET. The torodial form with
either relative or absolute addressing is provided for
convience.
Also the NVDET provides a mechanism for defining on the
screen fields with special attributes. For example,
characters entered into these fields may be displayed
with brighter intensity, highlighted by reverse video or
blinking, or protected from modification by the user.
This latter feature is one of the most heavily used for
applications where the DET displays a form to be filled
out by the user.
The definition of the NVDET uses Telnet option
subnegotiations to accomplish all of its functions.
Since none of the ASCII characters sent in the data
stream have been used to define these functions, the DET
option can be used in a "raw" or even "rare" mode. In
circumstances where the application program knows what
kind of terminal is on the other end, it can send the
ASCII characters required to control functions not
supported by the option or an implementation. In general
keeping all NVDET functions out of the data stream
provides better flexibility.
F__a_c_i_l_i_t_y_F__u_n_c_t_i_o_n_s (for detailed semantics see Section 5.)
IAC SB DET IAC SE
where is one 8-bit byte
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indicating the class of the facilities to be described,
and is a field of one or two 8-bit bytes
containing flags describing the facilities required or
desired by the sender. The bits of the facility maps are
numbered from the right starting at zero. Thus, if bit 2
is set the field will have a decimal value of 4. The
values of the field are as follows:
facility cmd: EDIT FACILITIES subcommand code: 1
facility map: bit numbers
Toroidal Cursor Addressing 6
Incremental Cursor Addressing 5
Read Cursor Address 4
Line Insert/Delete 3
Char Insert/Delete 2
Back Tab 1
Positive Addressing only 0
where:
If the Toroidal Cursor Addressing bit is set, the sender
requests or provides that the SKIP TO LINE and SKIP TO
CHAR subcommands be supported.
If the Incremental Cursor Addressing bit is set, the
sender requests or provides that the UP, DOWN, LEFT, and
RIGHT subcommands be supported.
If the Read Cursor bit is set, the sender requests or
provides the READ CURSOR subcommand.
If the Line Insert/Delete bit is set, the sender requests
or provides that the LINE INSERT and LINE DELETE
subcommands be supported.
If the Char Insert/Delete bit is set, the sender requests
or provides that the CHAR INSERT and CHAR DELETE
subcommands be supported.
If the Back Tab bit is set, the sender requests or
provides that the BACK TAB subcommand be supported.
If the Positive Addressing bit is set, then the sender is
informing the receiver that it can only move the cursor
in the positive direction. (Note: Terminals that have
this property also have a Home function to get back to
the beginning.)
facility cmd: ERASE FACILITIES subcommand code: 2
facility map: bit numbers
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Erase Field 4
Erase Line 3
Erase Rest of Screen 2
Erase Rest of Line 1
Erase Rest of Field 0
where:
If a bit of the facility map for this facility command is
set, the sender requests or provides the facility
indicated by the bit. For a more complete description of
each of these functions see the Erase Functions section
below.
facility cmd: TRANSMIT FACILITIES subcommand code: 3
facility map: bit numbers
Data Transmit 5
Transmit Line 4
Transmit Field 3
Transmit Rest of Screen 2
Transmit Rest of Line 1
Transmit Rest of Field 0
where:
If a bit of the facility map for this facility command is
set, the sender requests or provides the facility
indicated by the bit. For a more complete description of
each of these functions see the Transmit Functions
section below.
facility cmd: FORMAT FACILITIES subcommand code: 4
facility map: bit numbers
byte 0
Repeat 4
Blinking 3
Reverse Video 2
Right Justification 1
Overstrike 0
byte 1
Protection On/Off 6
Protection 5
Alphabetic-only Protection 4
Numeric-only Protection 3
Intensity 0-2
where:
If the Repeat bit is set the sender requests or provides
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the REPEAT subcommand.
If the Blinking bit is set, the sender requests or
provides the ability to highlight a string of characters
by causing them to blink.
If the Reverse Video bit is set, the sender requests or
provides the ability to highlight a string of characters
by "reversing the video image," i.e., if the characters
are normally displayed as black characters on a white
background, this is reversed to be white characters on a
black background, or vice versa.
If the Right Justification bit is set, the sender
requests or provides the ability to cause entries of data
to be right justified in the field.
If the Overstrike bit is set, the sender requests or
provides the ability to superimpose one character over
another on the screen much like a hard copy terminal
would do if the print mechanism struck the same position
on the paper with different characters.
If the Protection On/Off bit is set, the sender requests
or provides the ability to turn on and off field
protection.
If the Protection bit is set, the sender requests or
provides the ability to protect certain strings of
characters displayed on the screen from being altered by
the user of the terminal. Setting this bit also implies
that ERASE UNPROTECTED and TRANSMIT UNPROTECTED
subcommands (see below) are supported.
If the Alphabetic-only Protection bit is set, the sender
requests or provides the ability to constrain the user of
the terminal such that he may only enter alphabetic data
into certain areas of the screen.
If the Numeric-only Protection bit is set, the sender
requests or provides the ability to constrain the user of
the terminal such that he may only enter numerical data
into certain areas of the screen.
The three bits of the Intensity field will contain a
positive binary integer indicating the number of levels
of intensity that the sender requests or provides for
displaying the data. The value of the 3 bit field should
be interpreted in the following way:
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1 one visible intensity
2 two intensities; normal and bright
3 three intensities; off, normal, and bright
>3 >3 intensities; off, and the remaining levels
proportioned from dimmest to brightest intensity.
For the all of the above commands, if the appropriate bit
in is not set, then the sender does not
request or provide that facility.
E__d_i_t_i_n_g_F__u_n_c_t_i_o_n_s
IAC SB DET MOVE CURSOR IAC SE subcommand code: 5
where is an 8-bit byte containing a positive binary
integer representing the character position of the
cursor, is an 8-bit byte containing a positive binary
integer representing the line position of the cursor.
This subcommand moves the cursor to the absolute screen
address (x,y) with the following boundary conditions:
if x>M-1, set x=M-1 and send an ERROR subcommand
if y>N-1, set y=N-1 and send an ERROR subcommand
This describes a finite plane topology on the screen.
IAC SB DET SKIP TO LINE IAC SE subcommand code: 6
where is a positive 8-bit binary number.
This subcommand moves the cursor to the absolute screen
line y. x remains constant. For values of y>N-1
y = y mod N.
IAC SB DET SKIP TO CHAR IAC SE subcommand code: 7
where is a positive 8-bit binary number.
This subcommand moves the cursor to the absolute
character position x. y remains constant, unless x>M-1
in which case:
x' = (x mod M)
y' = (y+(x DIV N))
where x' and y' are the new values of the cursor.
These last two subcommands define a toroidal topology on
the screen.
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IAC SB DET UP IAC SE subcommand code: 8
IAC SB DET DOWN IAC SE subcommand code: 9
IAC SB DET LEFT IAC SE subcommand code: 10
IAC SB DET RIGHT IAC SE subcommand code: 11
These subcommands are provided as a convenience for some
terminals. The commands UP, DOWN, LEFT, and RIGHT are
defined as
UP: (x,y)=(x, y-1 mod N)
DOWN: (x,y)=(x, y+1 mod N)
LEFT: (x,y)=(x-1, y); if x=0 then x-1 = 0
RIGHT: (x,y)=(x+1 mod M, y) and y = y+1 if x+1>M-1
Note: DOWN, LEFT, and RIGHT cannot always be replaced by
the ASCII codes for linefeed, backspace, and space
respectively. The latter are format effectors while the
former are cursor controls.
IAC SB DET HOME IAC SE subcommand code: 12
This subcommand positions the cursor to (0,0). This is
equivalent to a MOVE CURSOR 0,0 or the sequence SKIP TO
LINE 0, SKIP TO CHAR 0. This subcommand is provided for
convenience, since most terminals have it as a separate
control.
IAC SB DET LINE INSERT IAC SE subcommand code: 13
This subcommand inserts a line of spaces between lines y
(the current line, determined by the position of the
cursor) and line y-1. Lines y through N-2 move down one
line, i.e. line y becomes line y+1; y+1 becomes y+2,
...; N-2 becomes N-1. Line N-1 is lost off the bottom of
the screen. The position of the cursor remains
unchanged.
IAC SB DET LINE DELETE IAC SE subcommand code: 14
This subcommand deletes line y where y is the current
line position of the cursor. Lines y+1 through N-1 move
up one line, i.e. line y+1 becomes line y; y+2 becomes
y+1; ...; N-1 becomes N-2. The N-1st line position is
set to all spaces. The cursor position remains
unchanged.
IAC SB DET CHAR INSERT IAC SE subcommand code: 15
This subcommand inserts the next character in the data
stream between the xth and x-1st characters, where x is
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the current character position of the cursor. The xth
through M-2nd characters on the line are shifted one
character positon to the right. The new character is
inserted at the vacated xth position. The M-1st
character is lost. The position of the cursor remains
unchanged.
IAC SB DET CHAR DELETE IAC SE subcommand code: 16
This subcommand deletes the character on the screen at
the x-th position. The x-th character is removed and the
characters x+1 through M-1 are shifted one character
position to the left to become the x-th through M-2nd
characters. The M-1st character position is left empty.
(For most terminals it will be set to a NUL or space.)
The cursor position remains unchanged.
IAC SB DET READ CURSOR IAC SE subcommand code: 17
This subcommand requests the receiver to send the present
position of the cursor to the sender.
IAC SB DET CURSOR POSITION IAC SE
subcommand code: 18
where and are positive 8-bit binary integers.
This subcommand is sent by a Telnet implementation in
response to a READ CURSOR subcommand to convey the
coordinates of the cursor to the other side. Note: x is
less than M and y is less than N.
IAC SB DET REVERSE TAB IAC SE subcommand code: 19
This subcommand causes the cursor to move to the previous
tab position. If none exists on the present line, the
cursor moves to the previous line and so on until a tab
is found or the address (0,0) is encountered. When field
protection is in effect the cursor moves to the beginning
of the preceding unprotected field.
T__r_a_n_s_m_i_t_F__u_n_c_t_i_o_n_s (For detailed semantics see Section 5.)
IAC SB DET TRANSMIT SCREEN IAC SE subcommand code: 20
This subcommand causes the terminal to transmit all
characters on the screen from position (0,0) to
(M-1,N-1). The cursor will be at (0,0) after the
operation is complete.
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RFC 731
IAC SB DET TRANSMIT UNPROTECTED IAC SE
subcommand code: 21
This subcommand causes the terminal to transmit all
characters in unprotected fields from position (0,0) to
(M-1,N-1). The unprotected fields are separated by the
field separator subcommand. The cursor will be at (0,0)
or at the beginning of the first unprotected field after
the operation is complete.
IAC SB DET TRANSMIT LINE IAC SE subcommand code: 22
This subcommand causes the terminal to transmit all data
on the yth line where y is determined by the present
position of the cursor. Data is sent from character
position (0,y) to the end-of-line or position (M-1,y)
whichever comes first. The cursor position after the
transmission is one character position after the end of
line condition or the beginning of the next line,
(0,y+1).
IAC SB DET TRANSMIT FIELD IAC SE subcommand code: 23
This subcommand causes the terminal to transmit all data
in the field presently occupied by the cursor. The
cursor position after the operation is complete is one
character position after the end of the field or, if that
position is protected, at the beginning of the next
unprotected field.
IAC SB DET TRANSMIT REST OF SCREEN IAC SE
subcommand code: 24
This subcommand causes the terminal to transmit all
characters on the screen from position (x,y) to (M-1,N-1)
or until the end of text. (x,y) is the current cursor
position. The cursor position after the operation is one
character position after the last text character, or
(0,0) if the last filled character position is (M-1,N-1).
IAC SB DET TRANSMIT REST OF LINE IAC SE
subcommand code: 25
This subcommand causes the terminal to transmit all
characters on the yth line from position (x,y) to the end
of line or (M-1,y) whichever comes first. (x,y) is the
current cursor position. The cursor position after the
operation is one character position after the last
character of the line or the first character of the next
line.
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IAC SB DET TRANSMIT REST OF FIELD IAC SE
subcommand code: 26
This subcommand causes the receiver to transmit the rest
of the characters in the field currently occupied by the
cursor. The cursor position after the operation is at
the beginning of the next field.
IAC SB DET DATA TRANSMIT IAC SE
subcommand code: 27
This subcommand is used to preface data sent from the
terminal in response to a user action or a TRANSMIT
command. The parameters and indicate the initial
position of the cursor. See the Transmit Subcommands
subsection in Section 5 for more details.
E__r_a_s_e_F__u_n_c_t_i_o_n_s
IAC SB DET ERASE SCREEN IAC SE subcommand code: 28
This subcommand causes all characters to be removed from
the screen. All fields regardless of their attributes
are deleted. The cursor position after the operation
will be (0,0). Most terminals set the erased characters
to either NUL or space characters.
IAC SB DET ERASE LINE IAC SE subcommand code: 29
This subcommand causes all characters on the yth line to
be removed from the screen, where y is the line of the
current cursor position. All fields regardless of their
attributes are deleted. The cursor position after this
operation will be (0,y). Note: This operation can be
easily simulated by the sequence: LINE DELETE, LINE
INSERT. However, the order is important to insure that
no data is lost off the bottom of the screen.
IAC SB DET ERASE FIELD IAC SE subcommand code: 30
This subcommand causes all characters in the field
occupied by the cursor to be removed. The cursor
position after the operation is at the beginning of the
field.
IAC SB DET ERASE REST OF SCREEN IAC SE
subcommand code: 31
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June 27,1977 NIC 40652
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This subcommand causes all characters from position (x,y)
to (M-1,N-1) to be removed from the screen. All fields
regardless of their attributes are deleted. The cursor
position after the operation is unchanged. This is
equivalent to doing an ERASE REST OF LINE plus a LINE
DELETE for lines greater than y.
IAC SB DET ERASE REST OF LINE IAC SE
subcommand code: 32
This subcommand causes all characters from position (x,y)
to (M-1,y) to be removed from the screen All fields
regardless of their attributes are deleted. The cursor
position after the operation is unchanged.
IAC SB DET ERASE REST OF FIELD IAC SE
subcommand code: 33
This subcommand causes all characters from position (x,y)
to the end of the current field to be removed from the
screen. The cursor position after the operation is
unchanged.
IAC SB DET ERASE UNPROTECTED IAC SE
subcommand code: 34
This subcommand causes all characters on the screen in
unprotected fields to be removed from the screen. The
cursor position after the operation is at (0,0) or, if
that position is protected, at the beginning of the first
unprotected field.
F__o_r_m_a_t_F__u_n_c_t_i_o_n_s
IAC SB DET FORMAT DATA IAC SE
subcommand code: 35
where is an 8-bit byte containing the
following flags:
Blinking 7
Reverse Video 6
Right Justification 5
Protection 3-4
Intensity 0-2
where:
If the Blinking bit is set, the following field of
characters should have the Blinking attribute
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applied to it by the receiver.
If the Reverse Video bit is set, the following field of
characters should be displayed by the receiver
with video reversed.
If the Right Justification bit is set, the input entered
into the field of characters should be right
justified.
The Protection field is two bits wide and may take on the
following values:
0 no protection
1 protected
2 alphabetic only
3 numeric only
The protection attribute specifies that the other side
may modify any character (no protection), modify no
characters (protected), enter only alphabetical
characters (A-Z, and a-z) (alphabetic only), or enter
only numerical characters (0-9,+,.,and -) (numeric only)
in the following field of bytes.
The Intensity field is 3 bits wide and should be
interpreted in the following way:
The values 0-6 should be used as an indication of the
relative brightness to be used when displaying the
characters in or entered into the following field
characters wide. The number of levels of brightness
available should have been obtained previously by the
Format Facility subcommand. The exact algorithm for
mapping these values to the available levels of intensity
is left to the implementors. A value of 7 in the
intensity field indicates that the brightness should be
off, and any characters in or entered into the field
should not be displayed.
is 2 bytes that should be interpreted as a
positive 16-bit binary integer representing the number of
characters following this command which are affected by
it.
Data sent to the terminal or the Using Host for unwritten
areas of the screen not in the scope of the count should
be displayed with the default values of the format map.
The default values are No Blinking, Normal Video, No
Justification, No Protection and Normal Intensity.
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This subcommand is used to format data to be displayed on
the screen of the terminal. The describes
the attributes that the field bytes wide should
have. This field is to start at the position of the
cursor when the command is acted upon. The next
displayable characters in the data stream are used to
fill the field. Subsequent REPEAT subcommands may be
used to specify the contents of this field. If the
sender specifies attributes that have not been agreed
upon by the use of the Format Facility subcommand, the
Telnet process should send an Error Subcommand to the
sender, but format the screen as if the bit had not been
set.
IAC SB DET REPEAT IAC SE
subcommand code: 36
where is a positive 8-bit binary integer.
is an 8-bit byte containing an ASCII character.
This subcommand is used to perform data compression on
data being transferred to the terminal by encoding
strings of identical characters as the character and a
count. The repeated characters may be part of a field
specified
IAC SB DET SUPPRESS PROTECTION IAC SE
subcommand code: 37
where may have the values of the Telnet
option negotiation:
251 WILL
252 WONT
253 DO
254 DONT
This subcommand is used to suppress the field protection
in a non-destructive manner. Many data entry terminals
provide the means by which protection may be turned on
and off without modifying the contents of the screen or
the terminal's memory. Thus, the protection may be
turned off and back on without retransmitting the form.
The default setting of the option is that protection is
on, in other words
IAC SB DET SUPPRESS PROTECTION WONT IAC SE
IAC SB DET SUPPRESS PROTECTION DONT IAC SE
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Negotiation of this subcommand follows the same rules as
negotiations of the Telnet options.
IAC SB DET FIELD SEPARATOR IAC SE subcommand code: 38
It is necessary when transmitting only the unprotected
portion of the screen to provide a means for delimiting
the fields. Existing DET's use a variety of ASCII
characters such as Tab, Group Separator, Unit Separator,
etc. In order to maintain transparency of the NVDET this
subcommand is used to separate the fields. Clearly, this
incurs rather high overhead. This overhead can be
avoided by using the Byte Macro Option (see Appendix 3).
M__i_s_c_e_l_l_a_n_e_o_u_s_C__o_m_m_a_n_d_s
IAC SB DET FN IAC SE subcommand code: 39
where: is one byte.
Many data-entry terminals provide a set of "function"
keys which when pressed send a one-character command to
the server. This subcommand describes such a facility.
The values of the field are defined by the user
and server. The option merely provides the means to
transfer the information.
IAC SB DET ERROR IAC SE
subcommand code: 40
where:
is a byte containing the subcommand code of
the subcommand in error.
is a byte containing an error code.
(For a list of the defined error codes see Appendix 2.)
This subcommand is provided to allow DET option
implementations to report errors they detect to the
corresponding Telnet process. At this point it is worth
reiterating that the philosophy of this option is that
when an error is detected it should be reported; however,
the implementation should attempt its best effort to
carry out the intent of the subcommand or data in error.
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3. Default and Minimal Implementation Specifications
D__e_f_a_u_l_t
WON'T DET -- DON'T DET
Neither host wishes to use the Data Entry Terminal option.
M__i_n_i_m_a_l_I__m_p_l_e_m_e_n_t_a_t_i_o_n
DET EDIT FACILITIES
DET ERASE FACILITIES
DET TRANSMIT FACILITIES
DET FORMAT FACILITIES
DET MOVE CURSOR
DET HOME
DET ERASE SCREEN
DET TRANSMIT SCREEN
DET FORMAT DATA
DET ERROR
In the case of formatting the data, the minimal
implementation should be able to support a low and high
level of intensity and protection for all or no
characters in a field. These functions, however, are not
required.
The minimal implementation also requires that the Output
Line Width and Output Page Size Telnet options be
supported.
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4. Motivation
The Telnet protocol was originally designed to provide a
means for scroll-mode terminals, such as the standard
teletype, to communicate with processes through the network.
This was suitable for the vast majority of terminals and
users at that time. However, as use of the network has
increased into other areas, especially areas where the
network is considered to provide a production environment
for other work, the desires and requirements of the user
community have changed. Therefore, it is necessary to
consider supporting facilities that were not initially
supported. This Telnet option attempts to do that for
applications that require data entry terminals.
This option in effect defines the Network Virtual Data Entry
Terminal. Although the description of this option is quite
long, this does not imply that the Telnet protocol is a poor
vehicle for this facility. Data Entry Terminals are rather
complex and varied in their abilities. This option attempts
to support both the minimal set of useful functions that are
either common to all or can be easily simulated and the more
sophisticated functions supplied in some terminals.
Unlike most real data entry terminals where the terminal
functions are encoded into one or more characters of the
native character set, this option performs all such controls
within the Telnet subnegotiation mechanism. This allows
programs that are intimately familiar with the kind of
terminal they are communicating with to send commands that
may not be supported by either the option or the
implementation. In other words, it is possible to operate
in a "raw" or at least "rare" mode using as much of the
option as necessary.
Although many data entry terminals support a variety of
peripheral devices such as printers, cassettes, etc. it is
beyond the scope of this option to entertain such
considerations. A separate option should be defined to
handle this aspect of these devices.
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5. Description
G__e_n_e_r_a_l_N__o_t_e_s
All implementations of this option are required to support a
certain minimal set of the subcommands for this option.
Section 3 contains a complete list of the subcommands in
this minimal set. In keeping with the Telnet protocol
philosophy that an implementation should not have to be able
to parse commands it does not implement, every subcommand of
this option is either in the minimal set or is covered by
one of the facility subcommands. An implementation must
"negotiate" with its correspondent for permission to use
subcommands not in the minimal set before using them. For
details of this negotiation process see the section below on
facility subcommands.
Most data entry terminals are used in a half duplex mode.
(Although most DET's on the market can be used either as
data entry terminals or as standard interactive terminals,
we are only concerned here with their use as DET's.) When
this option is used, it is suggested that the following
Telnet options be refused: Echo, Remote Controlled
Transmission and Echoing, and Suppress Go-Ahead. However,
this option could be used to support a simple full duplex
CRT based application using the basic cursor control
functions provided here. For these cases, one or more of
the above list of options might be required. (Support of
sophisticated interactive calligraphic applications is
beyond the scope of this option and should be done by
another option or the Network Graphics Protocol.)
In RFC 728, it was noted that a synch sequence can cause
undesired interactions between Telnet Control functions and
the data stream. A synch sequence causes data but not
control functions to be flushed. If a control function
which has an effect on the data immediately following it is
present in the data stream when a synch sequence occurs, the
control function will have its effect not on the intended
data but on the data immediately following the Data Mark.
The following DET subcommands are susceptible to this
pitfall:
CHAR INSERT
DATA TRANSMIT
FORMAT DATA
The undesired interactions are best avoided by the receiver
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of the synch sequence deleting these subcommands and all
data associated with them before continuing to process the
control functions. This implies that the Data Mark should
not occur in the middle of the data associated with these
subcommands.
F__a_c_i_l_i_t_y_S__u_b_c_o_m_m_a_n_d_s
These four subcommands are used by the User and Server
implementations to negotiate the subcommands and attributes
of the terminal that may be utilized. This negotiation can
be viewed as the terminal (User Host) indicating what
facilities are provided and the Server Host (or application
program) indicating what facilities are desired.
W__h_e_n_S__e_n_t:_ A Server Telnet implementation using the DET
option must send a facility subcommand requesting the use of
a particular subcommand or terminal attribute not in the
minimal implementation before the first use of that
subcommand or attribute. The User Telnet implementation
should respond as quickly as possible with its reply.
Neither the User nor Server are required to negotiate one
subcommand at a time. Also, a Telnet implementation
responding to a facility subcommand is not required to give
permission only for that subcommand. It may send a format
map indicating all facilities of that class which it
supports. However, a Telnet implementation requesting
facilities must send a facility subcommand before its first
use of the subcommand regardless of whether earlier
negotiations have indicated the facility is provided. The
facility cannot be used until a corresponding facility
subcommand has been received. There are no other
constraints on when the facility subcommands may be sent.
In particular, it is not necessary for an application to
know at the beginning of a session all facilities that it
will use.
A__c_t_i_o_n_W__h_e_n_R__e_c_i_e_v_e_d:_ There are two possible actions that
may be taken when a facility subcommand is received
depending on whether the receiver is a requestor or a
provider (User).
Requestor: When a facility subcommand is received by a
requestor and it is in the state of Waiting for a Reply, it
should go into the state of Not Waiting. It should then
take the facility map it had sent and form the logical
intersection with the facility map received. (For the
Intensity attribute, one should take the minimum of the
number received and the number requested.) The result
indicates the facilities successfully negotiated. Note: if
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the receiver is not in the Waiting for Reply state, then
this is the provider case described next.
Provider: When a facility subcommand is received, it
should send a facility subcommand with a facility map of the
facilities it provides as soon as possible. It should then
determine what new facilities it is providing for the
Requestor by forming the logical intersection of the
facility map received and the one sent.
Note: Although in most cases the requestor will be the
Server Host and the provider will be the User Host
supporting the terminal, this distinction may not always be
true.
T__r_a_n_s_m_i_t_S__u_b_c_o_m_m_a_n_d_s
There are two kinds of transmit subcommands: those used to
request that data be sent to the requestor, and one to
preface data sent to the requestor. The first kind allow
the requestor to control when, from where and to some degree
how much data is transmitted from the terminal. Their
explanation is straightforward and may be found in Section
2.
Data may be sent from the terminal as a result of two
events: the user of the terminal caused the transmission or
in response to a transmit subcommand. Some programs may
wish to know from where on the screen the transmission
began. (This is reasonable, since the terminal user may
move the cursor around considerably before transmitting.)
Other programs may not need such information. The DATA
TRANSMIT subcommand is provided in case this function is
needed. When used this subcommand prefaces data coming from
the terminal. The parameters and give the screen
coordinates of the beginning of the transmission. must
be less than or equal to M-1 and must be less than or
equal to N-1. It is assumed that all data between this DATA
TRANSMIT and the next one starts at the coordinates given
by the first subcommand and continues filling each line
thereafter according to the constraints of the screen and
the format effectors in the data. Thus an intelligent or
sloppy user-host DET implementation (depending on your point
of view) need only include a DATA TRANSMIT subcommand when
the new starting point is different from the last ending
point.
6. Sample Interaction
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The nomenclature of RFC 726 will be used to describe this example. To
quote that RFC:
"S:" is sent from serving host to using host.
"U:" is sent from using host to serving host.
"T:" is entered by the terminal user.
"P:" is printed on the terminal.
Text surrounded by square brackets([]) is commentary. Text
surrounded by angle brackets (<>) is to be taken as a single unit.
E.g, carriage return is , and the decimal value 27 is
represented <27>.
We assume that the user has established the Telnet connection,
logged on, and an application program has just been started either
by the user directly or through a canned start up procedure. The
presentation on the page is meant to merely group entities together
and does not imply the position of message boundaries. One should
assume that any part of the dialogue may be sent as one or many
messages. The first action of the program or Telnet is to
negotiate the DET option:
S:
U:
S: [First negotiate the screen
size. In this case we are
U: asking the user the size of the
terminal. This could have been
U:<25> done before the DET option was
negotiated.]
S:<0>
S:
U:<80> [Defines the screen to be 25
lines by 80 characters.
S:<0> The server may use this
information when formatting
the screen.]
S: [Now set the terminal attributes.]
U:
S: [Erase the screen and
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start sending the form.]
<0>
<5>Name:
<0><1>
Protection=1, Intensity=1><0>
<8>
Address:
<0><4>
Protection=1, Intensity=1><0>
<17>
Telephone number:
<32><4>
Protection=1, Intensity=1><0>
<24>
Social Security Number:
[Establish a field that doesn't
display what is typed into it.]
<0><11><32>
<5> [Get permission to use Blinking
<0> Attribute.]
U:
S:<0><29>
Your SSN will not be printed.
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The previous exchange has placed a form on the screen that looks like:
Name:
Address:
Telephone Number: Social Security Number:
"Your SSN will not be printed."
where the quoted string is blinking.
The terminal user is now free to fill in the form provided. He
positions the cursor at the beginning of the first field (this usually
is done by hitting the tab key) and begins typing. We do not show this
interaction since it does not generate any interaction with the User
Telnet program or the network. After the terminal user has completed
filling in the form, he strikes the transmit key to send the unprotected
part of the form, but first the User Telnet program negotiates the Byte
Macro Option to condense the Field Separator subcommand:
U: [Negotiate Byte Macro Option.]
S: [Define decimal 166 to be the
Field Separator subcommand
U: (see Appendix 3)]
<166><6>
S:<166> [The server accepts the macro.]
U:<0><6>
John Doe <166> 1515 Elm St., Urbana, Il 61801
<166> 217-333-9999 <166> 123-45-6789 <166>
S:
Thank you.
And so on.
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Appendix 1 - Subcommands, opcodes and syntax
1 EDIT FACILITIES
2 ERASE FACILITIES
3 TRANSMIT FACILITIES
4 FORMAT FACILITIES
5 MOVE CURSOR
6 SKIP TO LINE
7 SKIP TO CHAR
8 UP
9 DOWN
10 LEFT
11 RIGHT
12 HOME
13 LINE INSERT
14 LINE DELETE
15 CHAR INSERT
16 CHAR DELETE
17 READ CURSOR
18 CURSOR POSITION
19 REVERSE TAB
20 TRANSMIT SCREEN
21 TRANSMIT UNPROTECTED
22 TRANSMIT LINE
23 TRANSMIT FIELD
24 TRANSMIT REST OF SCREEN
25 TRANSMIT REST OF LINE
26 TRANSMIT REST OF FIELD
27 DATA TRANSMIT
28 ERASE SCREEN
29 ERASE LINE
30 ERASE FIELD
31 ERASE REST OF SCREEN
32 ERASE REST OF LINE
33 ERASE REST OF FIELD
34 ERASE UNPROTECTED
35 FORMAT DATA
36 REPEAT
37 SUPPRESS PROTECTION
38 FIELD SEPARATOR
39 FN
40 ERROR
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Appendix 2 - Error Codes
1 Facility not previously negotiated.
2 Illegal subcommand code.
3 Cursor Address Out of Bounds.
4 Undefined FN value.
4 Can't negotiate acceptable line width.
5 Can't negotiate acceptable page length.
6 Illegal parameter in subcommand.
7 Syntax error in parsing subcommand.
8 Too many parameters in subcommand.
9 Too few parameters in subcommand.
10 Undefined parameter value
11 Unsupported combination of Format Attributes
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Appendix 3 - Use of the Byte Macro Option
One of the major drawbacks of the DET option is that because the
functions are encoded as Telnet option subnegotiations a fairly
high overhead is incurred. A function like Character Insert which
is encoded as a single byte in most terminals requires six bytes in
the DET option. Originally the only other solution that would have
accomplished the same transparency that the use of subcommands
provides would have been to define additional Telnet control
functions. However, since this would entail modification of the
Telnet protocol itself, it was felt that this was not a wise
solution. Since then the Telnet Byte Macro Option (RFC 729) has
been defined. This option allows the user and server Telnets to
map an arbitrary character string into a single byte which is then
transferred over the net. Thus the Byte Macro Option provides the
means for implementations to avoid the overhead for heavily used
subcommands. The rest of this appendix suggests how the Byte Macro
Option should be applied to the DET option.
In keeping with the specification of the Byte Macro Option, macro
bytes will be chosen from the range 128 to 239. For the DET
option, it is suggested that macro bytes be chosen by adding the
subcommand code to 128. In addition, an unofficial DET subcommand
might be defined indicating that each side was willing to support
macro bytes for all subcommands (but not necessarily support all of
the subcommands themselves) according to this algorithm. This
subcommand would be:
IAC SB DET DET-MACRO IAC SE
subcommand code: 254
where may have the values of the Telnet option
negotiation:
251 WILL
252 WONT
253 DO
254 DONT
This subcommand is sent by a Telnet implementation to indicate its
willingness to adopt byte macros for all of the DET subcommands
according to the following algorithm:
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The macro byte for subcommand i will be i+128 and will represent
the following string for parameterless subcommands:
IAC SB DET